The invention relates generally to electrical machines. More particularly, the invention relates to a laminated rotor assembly for an electrical machine.
Laminated rotors assemblies are used in electric generators, electric motors and electric starters for turbomachines such as Auxiliary Power Units (APUs) and Integrated Power Units (IPUs), and propulsion systems such as turbofan systems, turboprop systems and turboshaft systems. Typically, these electrical machines are external to their turbine engines, contain their own bearing systems and, for high-speed turbine engines, are driven by gears.
Integration of the electrical machine with the turbine engine is desirable because gears and bearings can be eliminated and construction simplified. However, such integration requires the laminated rotor assembly to have a high relative stiffness, adequate torque transfer, and an ability to maintain piloting and balance at high operating speeds.
Increasing the diameter of the rotor assembly in the electrical machine is also desirable because tip speed can be increased and axial length can be reduced, all without reducing power output of the electrical machine. Reducing the axial length of the rotor assembly also allows dynamic performance to be improved.
However, increasing the diameter of the rotor assembly is difficult to do. The rotor assembly includes a stack of laminations, which are secured to a shaft by way of an interference fit. As the diameter of the lamination stack is increased, the interference fit becomes larger. The larger interference causes problems during assembly. A larger interference fit needs a larger axial load to clamp the lamination stack. Yet only so much force can be applied to the shaft to force it into the lamination stack's bore. The larger interference fit also causes problems during high temperature/high speed operation of the rotor assembly. If the interference fit between the laminations and the shaft is lost, the rotor assembly suffers large amounts of unbalance and vibration that could cause damage.
Increasing the diameter of the rotor assembly is even more difficult to do for an electrical machine that is integrated with the turbine engine. Loss of the interference fit reduces the ability of the rotor assembly to maintain piloting and balance at high speeds. Moreover, the laminated rotor assembly has low relative stiffness. Yet the rotor assembly of an integrated electrical machine needs good balance, proper piloting and a relatively high stiffness to maintain proper dynamic characteristics.